In addition to controlling host sensitivity to the biological effects of endotoxin, the Lps gene locus is an important regulator of murine macrophage function and the immune physiology of the intact animal. Macrophages from mice that possess the defective allele of this locus (Lpsd), exhibit a number of in vivo abnormalities including a loss of surface receptors and poor tumoricidal capacity. These mice also exhibit abnormal macrophage function in vivo as manifested by their marked susceptibility to several infectious agents such as Salmonella typhimurium. Moreover, there is a connection between these macrophage abnormalities and endotoxin unresponsiveness since maneuvers that increase the state of macrophage activation in vitro (T-cell lymphokines) or in vivo (BCG infection), render Lpsd cells or mice endotoxin sensitive. In the proposed studies, the mechanisms by which the Lps gene locus regulates macrophage function and endotoxin responsiveness in vitro and in vivo will be investigated. In addition, the macrophage defects caused by the Lpsd allele will be utilized to characterize the sequence of steps involved in the differentiation of resting monocytes to activated macrophages; to determine the intra and inter cellular signals involved in this process, and to physicochemically analyze and partially purify the relevant lymphokines. Control of surface receptors for Fc and C3b will be compared in macrophages from C3H/HeJ (Lpsd) and C3H/HeN (Lpsn) mice. T-cell derived lymphokine(s) will be analyzed for their ability to reverse the macrophage receptor defects as well as later parameters of activation including the ability to phagocytose particles via C3b receptors or to respond to endotoxin. These lymphokines will then be purified in a sequential fashion by standard column chromatographic, affinity, and electrophoretic techniques. The role of specific cyclic nucleotides or metabolites of arachidonic acid as inducers of macrophage differentiation will be studied. The in vivo role of differentiated macrophages in determining endotoxin sensitivity will be studied using the adoptive transfer of specific cell types into endotoxin-unresponsive mice. Finally the relationship between the observed macrophage defects and susceptibility to infection will be studied in vivo and in vitro by analyzing the killing of gram-negative organisms.